This invention relates to the use of an adhesive containing at least two components A and B for the production of film laminates. The invention also relates to the use of this radiation-curing laminating adhesive for the production of film laminates.
In the field of adhesives, particularly in the lamination of web-form materials, there is an increasing demand for short cure times and a shorter response time of the adhesives used for lamination. Conventional systems widely available on the market are generally based on polyurethane, acrylate or epoxy binders which cure by relatively slow crosslinking through reaction with added hardener or with moisture. The usual cure time for commercially available systems such as these is about 4 to about 21 days. However, a cure time as long as this to achieve maximal strength of the film laminates is generally not desirable.
Film laminates are exposed in their production, processing and use to a number of stresses which, typically, do not occur with other bonded materials or do not occur to the same extent as they do in film laminates. In the production of film laminates, differentxe2x80x94sometimes completely differentxe2x80x94materials with, in addition, a different surface structure are bonded to one another. They are generally web-form materials of, for example, paper, plastics, plastics coated by vapor deposition with metals or metal oxides, more particularly transition metals oxides, or metal foils, more particularly aluminium foils.
During their production, processing and use, the film laminates are exposed to a number of mechanical stresses which impose stringent demands on the mechanical properties of the adhesive establishing the bond between the materials. Since the web-form materials to be bonded are normally materials with high flexibility which are constantly exposed to tensile and flexural stresses during production, processing and use, the adhesive itself has to possess sufficiently high flexibility to be able to withstand the stresses occurring without damage or failure of the adhesive bond.
In addition, however, the adhesive is also expected to show high peel strength to be able to withstand tensile stresses applied perpendicularly to the laminate surface without separation of the film laminate.
In addition, the adhesive is also generally expected to satisfy various criteria in regard to crystallization behavior and discoloration which exceed the performance features of adhesives for conventional applications. For example, in the bonding of transparent plastic films, the film laminate is also expected to remain transparent without clouding through crystallization of the adhesive. In addition, the adhesive must not have any tendency to form colored secondary products, even in the event of prolonged storage of the film laminate, for example under UV light.
In addition, film laminates are expected to show high heat resistance after only a short time. This property is particularly important when film laminates are to be used for packaging products while they are still hot, for example with a view to shortening production and filling cycles. However, the feature of heat resistance is also of importance when, for example, materials already at least partly wrapped in the film laminate are to be subjected to heating.
A quality criterion increasing in significance for film laminates is the substantial absence of xe2x80x9cmigratesxe2x80x9d. Migrates are understood to be low molecular weight constituents of the film laminate which, on the one hand, are not immobile within the laminate, i.e. are capable of migrating within the laminate, and which on the other hand are capable of diffusing from the laminate into the material wrapped in the laminate. Since low molecular weight constituents such as these can affect the physical health of living beings, more particularly human beings, there is a need to provide substantially migrate-free film laminates.
Components capable of radical polymerization by irradiation in adhesives are known. However, the disadvantage of such radically polymerizing adhesives is that they generally have to be cured under inert conditions because atmospheric oxygen acts as an inhibitor. This is achieved, for example, by irradiating the material to be polymerized in an inert gas atmosphere of, for example, nitrogen or argon. The disadvantage of this procedure is that it can involve significant outlay on equipment to guarantee the necessary inert conditions.
DE-U 94 20 640.6 relates to radiation-curing compositions containing OH-terminated polyurethanes, an epoxy compound and a photoinitiator. The document in question describes a single-stage, radiation-curing adhesive composition which is characterized both by high initial adhesion and by high ultimate adhesion when it is used as a laminating adhesive.
EP-A 0 688 804 describes multicomponent, cationically curing epoxy compositions and a process for curing these compositions. It describes cationically curing epoxy compositions in various embodiments which generally contain as basic constituents a mixture of compounds that form Lewis acids and/or Brxc3x6nsted acids under irradiation, cationically polymerizable monomers containing epoxy groups and at least one other constituent selected from the group of flexibilizing agents, retarders, radical-polymerizable monomers, accelerators and modifiers. Alcohols and glycols with a molecular weight of at least 200 to 20,000 g/mole are mentioned as flexibilizing agents.
DE-A 43 40 949 describes cationically curing epoxy compositions and their use. This document mentions a photoinitiated, cationically curing epoxy composition which contains at least one retarder, at least one accelerator, at least one ferrocenium complex salt and at least one cyclo-aliphatic compound containing epoxy groups together with typical auxiliaries and additives.
The adhesives representing the prior art are generally attended by the disadvantage that they do not satisfy all the requirements which an adhesive used for the production of film laminates is expected to meet. Thus, although quick-curing adhesives can be produced from epoxy compounds and polyurethane polyols, their heat resistance is not as good as it should be, for example, for the preparation of foods or the sterilization of medical instruments.